The present invention relates to a multilayered structural body and a method for the preparation of a structural body such as a vessel having a multilayered wall. More particularly, the invention relates to a multilayered structural body imparted with low permeability to gases utilizing a styrene-based resin retaining excellent moldability of such resins in general and method for the preparation of a vessel having such a multilayered wall.
As is known, plastic resins are widely used as a wrapping material by virtue of their excellent properties such as absence of hygenic problems, good protection for the wrapped material, adaptability to shaping into desired forms, handiness, low costs and capability of giving various kinds of light-weight or see-through vessels or bags. In particular, plastic films have an extremely great and rapidly growing demand as a low-cost wrapping material of foods capable of excellently keeping the freshness of the foods wrapped therewith.
Polystyrene-based resins as a class of thermoplastic resins have excellent moldability at an elevated temperature and high rigidity of shaped articles thereof so that they are widely used as a wrapping material of foods. On the other hand, copolymeric resins consisting of the moieties of ethylene and vinyl alcohol have low permeability to gases so that they are also widely used as a wrapping material of foods, especially, when the foods are desired to be protected from the influences of the atmospheric oxygen or moisture.
A multilayered laminate consisting of layers of a polystyrene and an ethylene-vinyl alcohol copolymer, referred to as an EVOH copolymer hereinafter, is a well known material and various types have been proposed thereof. A problem in the application of such a multilayered laminate is that, since the optimum molding temperature differs between a polystyrene-based resin and an ethylene-vinyl alcohol copolymer as a matter of course, such a laminate cannot be molded into a structural body such as a vessel by molding at a relatively low molding temperature which might be suitable for molding of a polystyrene-based resin sheet per se.
Japanese Patent Kokai 52-112120 discloses a five-layered laminate consisting of two layers of a polystyrene-based resin and a layer of an EVOH copolymer sandwiched between the polystyrene resin layers with a layer of an adhesive resin intervening between the layer of the EVOH copolymer and each of the polystyrene resin layers. No consideration, however, is made in this disclosure on the moldability of the laminate relative to the constitution of the laminated sheets.